The developmental direction of communication systems is compatible with multiple standards and multiple wave bands. In the case of mobile communication systems, four 3G standards, WCDMA, CDMA2000, TD-SCDMA, and WIMAX, have been commercialized; meanwhile the old 2G networks still cover most areas. In the case of wireless local area networks, 802.11 series and Hiperlan etc. coexist. In the case of family networks, IEEE 802.15 series, Bluetooth, and zigbee etc. coexist. In the case of simplex communication systems such as digital TV broadcasting, signals are transmitted using different standards in different wave bands. In these communication systems, a receiver needs to support a plurality of clock signals and be able to process signals with different spectrums. To simplify structure, resources are shared. As a result, software radio solutions using digital signal processing technologies are more and more popular. In these solutions, the received analogue signal is converted into a digital signal. Then the digital signal is processed in digital domain to depress noise and/or demodulate. After that, the processed digital signal is converted back into an analogue signal. A data rate conversion device having adjustable bandwidth is necessary in software radio solutions.